Method and apparatus for incorporating additives in a melt

ABSTRACT

A method and apparatus for incorporating a normally oxidizable and/or volatile additive into a molten metal bath disposed in a vessel with a relatively gas tight cover thereover. A gas which is normally nonreactive with the additive is introduced, in one form of the invention, through a porous refractory plug in the bottom of the vessel and this gas bubbles up through the molten metal agitating the metal and fills the space above the metal and under the cover with at least a small positive pressure of the gas. This gas substantially excludes air from the space above the metal. A gas outlet is provided in the cover and is controllable for the purpose of regulating the outflow of gas to atmosphere thereby regulating the gas pressure above the metal surface. An additive container or hopper directly communicates by way of a valve with the space above the metal so that when desired an additive can be introduced into the molten metal. Cooling means are disclosed to condense any volatilized additive.

United States Patent [72] Inventors William H. Moore Meadow Laue,Purchase, N.Y. 10577; Harry H. Kesler. 7 Dromara Road. Ladue,

Mo.64758 2n AppLNo. 791,062 1221 Filed Jan. 14,1969

[4S] Patented Aug-10,1971

[54] METHOD AND APPARATUS FOR INCORPORATING ADDITIVES IN A MELT FOREIGNPATENTS 555,980 4/1958 Canada 898,986 6/1962 Great Britain PrimaryExaminer-Gerald A. Dost Anomey-Woodling. Krost, Granger and Rustthereover. A gas which is normally nonreactive with the additive isintroduced, in one form of the invention, through a porous refractoryplug in the bottom of the vessel and this gas bubbles up through themolten metal agitating the metal and fills the space above the metal andunder the cover with at least a small positive pressure of the gas. Thisgas substantially excludes air from the space above the metal. A gasoutlet is provided in the cover and is controllable for the purpose ofregulating the outflow of gas to atmosphere thereby regulating the gaspressure above the metal surface. An additive container or hopperdirectly communicates by way of a valve with the space above the metalso that when desired an additive can be introduced into the moltenmetal. Cooling means are dis closed to condense any volatiiizedadditive.

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METHOD AND APPARATUS FOR INCORPORATING ADDITIVES IN A MELT Our inventionrelates to an improved method of introducing volatile, oxidizable andcombustible alloys into-a ferrous or nonferrous metal. For example, itrelates to the introduction of magnesium, calcium, lithium, aluminum,antimony, zinc, sodium and other alloys, for the purposes of deoxidationor other special effects, in such a way that the additive alloy isreadily incorporated in the melt without excessive loss due tovolatilization, oxidation, etc.

Our invention is more particularly directed towards incorporatingelemental magnesium or concentrated magnesium alloys into cast irons forthe purpose of making nodular graphite cast iron.

An object of this invention is to provide a means and method ofincorporating alloys into the melt so as to increase the efficiency ofrecovery.

Another object is to allow the use of concentrated magnesium alloys oreven metallic magnesium by addition directly to the melt, for thepurpose of making nodular irons.

Another object is to allow other highly oxidizable alloys to be usedeffectively in both ferrous and nonferrous melts.

Another object is to allow volatile alloys to be used effectively inferrous and nonferrous melts.

Still another object is to decrease the degree of oxidation of the metalbath during the process of degassing or purifying of the melt.

Other objects of this invention will become readily apparent on readingthe specification and drawings in which:

FIG. 1 is a diagrammatic view of the apparatus of the preferredembodiment of this invention in which 1 is a refractory-lined vessel forholding molten metal, the surface of which is indicated at 6. A cover 2is held in position by clamps 3 fitted with an alloy hopper 9 having'acover 11 and a controllable valve 10. A controllable outlet 7 is fittedwith a valve control 8 and a cooling coil 20 surrounds the outlet. Aporous refractory plug assembly 4 is supplied by a gas line 5. Apressure guage 12 for the purpose of ensuring a positive pressure in thechamber is attached to the core;

FIG. 2 is essentially the same arrangement as FIG. 1 showing a metalinlet 14 and a metal outlet 13, enabling metal to be introduced andexhausted from the apparatus continuously. A cooling gas supply 21 isprovided in the cover for introducing a gas under pressure which uponexpansion creates a cooling effect in the chamber above the moltenmetal;

F IG. 3 is another arrangement of a suitable apparatus for thisinvention, having the same features, but introducing the gas directlythrough the cover through the gas pipe and being fitted with arefractory rotatable paddle 22 driven from a motor 23 -through two gears24; and

FIG. 4 is still another suitable apparatus for this invention comprisinga refractory-lined vessel 1 with a porous plug 4, a gas inlet 5 and acover 2 with a refractory bottom 25 projecting into and below thesurface of the metal 6. An alloy hopper 9 is provided which has a cover11, and a valve 10. A gas outlet 7 is located in the cover 2 and alsohas a controllable valve 8. In this embodiment the cover 2 may becarried by a hoist or other suitable means so it can be removed readilyfrom the vessel 1.

The addition of certain alloys to molten metal for the purpose ofdeoxidation, as with phosphorus in bronzes; or nudularization, as withmagnesium in nodular iron; or deoxidation, as with aluminum and calciumin steels, has always been a matter of extreme difficulty. Usually,alloys or elements which are strong deoxidants, by virtue of the factthat they combine readily with oxygen, also tend to burn directly in theair at the temperature of the metal to which they are being added; thus,their effectiveness in deoxidizing the melt is hampered by the fact thatthey burn at the surface, where the melt is exposed to air.

In the case of making nodular iron, for example, by adding magnesium tothe melt an attempt has been made to overcome this problem of magnesiumloss by various means; for example, it is common to use a diluted alloyof magnesium, such as nickel magnesium or magnesium ferrosilicon, in aneffort to slow down the rate of magnesium oxidation and thereby obtainmore recovery of magnesium in the melt. This has the disadvantage thatthe elements that may or may not be required were always added with themagnesium and larger alloy additions had to be made, whereas, in actualfact, only a relatively small amount of magnesium was actually requiredto be retained in the melt. For example, a recovery of magnesium of 10percent or 20 percent is quite common in the normal procedures used formaking nodular cast iron.

Other methods have been used, which consist of placing the magnesiumalloy in the bottom of the ladle and covering it with steel chips orferrosilicon, in such a way that the surface .of the alloy may berapidly covered with molten metal and,

therefore, decrease the burning or oxidation in the air. In such a casethe magnesium volatizes in the bottom of the bath and rises through themetal, where it burns quite violently at the point where the metal bathsurface comes in contact with the oxygen in the air.

Another popular method is to inject metallic magnesium or magnesiumalloy under the surface of the melt by using a suitable lance or tube.Here again, magnesium rises through the metal to the surface in the formof vapor and burns at the surface of the metal, where it contacts theoxygen in the air. Recovery by this method is usually on the order of 10percent of the magnesium added.

Another method is to use magnesium alloys which are relatively diluteand place these into a refractory bell, which is then plunged under thesurface of the metal. This method also allows magnesium vapor to risethrough the metal bath but here again, this magnesium burns violently oncontact with the air at the surface of the metal bath.

The usual method preferred by those skilled in the art in adding anyoxidizable low-melting point and perhaps volatile alloy to the bath, isto somehow plunge it under the surface or cover it quickly with metal,so as to prevent undue loss of the alloy.

We have discovered a means of introducing alloys of this type into meltsin such a way that oxidation is entirely eliminated or is reduced to theextent where the alloy addition is considerably more effective. We dothis by special means and method, whereby we provide for agitation inthe metal bath, exclusion of oxygen or air from the surface of the bathand a means for introducing the alloy into the melt through anatmosphere which is essentially inert or nonreactive with the materialsbeing added.

in the preferred embodiment of our invention we introduce the inert gasthrough a porous plug or a lance, which allows the gasto enter the metalat the bottom of the bath and provides for agitation of the bath withthe inert gas.

Ne contain the bath in a suitable vessel, which is fitted with arelatively airtight cover and a gas escape valve, with a controllableoutlet.

This cover effectively encloses the area above the bath, which wouldnormally be in contact with air. The gas bubbling through the bath,ejects air from the enclosure at the top of the bath and replaces itwith an inert atmosphere, such as nitrogen, argon or with any other gaswhich may be used, such as natural gas, carbon monoxide, etc., whichdoes not contain oxygen. Gases which provide special atmospheres. suchas chlorine, fluorine, etc., may also be used. We find that a positivein the chamber of at least l-inch water gauge is usually sufficient toensure removal of all air from the chamber. We prefer to work in thepressure range of a 1-24 inches of water but may go higher particularlywhere volatile alloys are involved. ln practicing our invention we startthe gas flow setting up agitation and allow sufficient time for theatmosphere pocket above the metal to be exhausted and replaced by theinert gas we are using. We then add the alloy to the surface of thebath, through a hopper which we attach to the cover, above the bath, insuch a way that the alloy never contacts any air. The alloy, thus added,falls to the surface of the metal and is incorporated in the melt,without any opportunity for it to contact air and, therefore, becomeoxidized.

If the alloy is excessively volatile, we supply cooling means at thecontrollable exhaust, so as to condense the alloy and allow it to fallback into the bath, much as would be the case with a reflux condenser.We have also found that the injection of a cooling gas under pressuresuch as CO or liquid nitrogen in the upper part of the chamber causesvapor to liquify and drop back into the molten metal.

While we prefer to use gas for agitation, we find that we are also ableto provide agitation by mechanical means by oscillating or shaking theladle with the airtight covers or by means of a rotating refractorypaddle immersed into the bath.

Where we use such mechanical means, we replace the air in thecompartment above the metal by injecting inert gas into this compartmentand allowing it to escape through the escape vent.

By maintaining positive pressure in this compartment, either throughdirect injection of inert gas or by injecting inert gas through a lanceor a porous plug, we are able to effectively exclude air from thecompartment and thereby promote conditions necessary for efficientincorporation of the alloy or ele ment into the melt.

' While the subsurface injection of inert gas as a carrier into the meltor the use of inert gas for agitation through the porous plug are bothmethods known to those skilled in the art, it has never been realizedthat it is necessary to confine this inert gas to the surface of themetal, so as to effectively exclude air particularly for additives to beintroduced through direct addition to the molten metal surface. Commonusage of all these' methods has neglected the surface of the metal bath,which is invariably in contact with the air and, because of this, it hasnever been possible to obtain fully effective incorporation ofoxidizable alloys into the bath.

In the practice of this invention we would refer to FIG. 1 of thedrawings. A melt, to which the oxidizable alloy is to be added is placedin the refractory-lined vessel 1 and the cover 2 is clamped intoposition with'the clamps 3. The alloy to be added is placed in thehopper 9 with the valve 10 in the closed position and with the cover 11in place.

Gas is introduced through the metal by means of the porous refractoryplug 4 and by opening the control on the gas inlet 5. The metal surface6 is brought to a suitable degree of agitation and valve control 8 ispartially opened, so as to leave a positive pressure by gauge 12) in thechamber formed between the cover 2 and the ladle 1.

Gas is blown for a period of l to 2 minutes, to completely replace theair in the aforesaid chamber. At this time the valve 10 is opened andthe additive alloy is allowed to fall on the surface of the metal 6,whereby it c becomes effectively incorporated into the melt. As soon assufficient time has occurred for incorporation, which is usually fromten to 30 seconds, the gas flow through the gas inlet pipe 5 is stopped,the cover 2 is removed from the refractory-lined vessel and the metal isready for casting. The cooling coil may be utilized if it is necessaryto condense any vaporized additive.

In producing nodular iron by using this apparatus and this method, wehave been able to obtain recoveries of magnesium as high as 90 percentof the magnesium added. Where this magnesium consists of a relativelydilute alloy; for example, in treating a bath, an addition of 1% percentof a 5 percent magnesium ferrosilicon alloy, giving a total magnesiumadded of 0.075 percent, resulted in a final treated bath having amagnesium content of 0.06 percent. This corresponds to an 80 percentrecovery of magnesium.

During the treatment. no violent flare or burning of magnesium wasobserved and the gas used for agitation was nitrogen passed through themetal at a pressure of about 30 p.s.i., and at a flow rate of eightcubic feet per ton of metal treated.

In producing nodular iron with a more concentrated magnesium alloy as,for example, a nickel magnesium alloy containing 20 percent magnesium,it was possible to obtain a recovery of 70 of the magnesium added to'themetal. With pure magnesium lumps as the alloy we have obtainedrecoveries of 50 percent.

In using the apparatus of the invention for nonferrous metals, we havefound it possible, for example, to get very close to percent recovery ofadditives, such as zinc.

ln treating cast iron, we have also found it possible to get very highrecoveries with elements such as lithium and calcium. Calcium, we foundto be particularly interesting, in that calcium is normally verydifficult to add to the metal, because of its tendency to oxidize at thesurface, forming refractory slag in the metal. In our invention, theformation of this oxidized slag was completely prevented and calcium waseffectively absorbed into the metal.

As mentioned hereinabove, FIG. 2 differs slightly from FIG. 1 in thatinlet and outlet passages are provided so as to provide continuous metaltreatment and in this embodiment a source of cooling gas 21 is providedto condense any vaporized additive in the space under the cover 2, inaddition to the gas supply 5. In FIG. 3 the inert gas atmosphere isprovided by gas introduced through conduit 5 directly into the spaceunder cover 2 and mechanical stirring is accomplished by stirrer 22which extends into the molten metal and driven by motor 23 throughgearing 24. The embodiment of FIG. 4 differs from the other showings inthe construction of the cover 2. Instead of the clamps 3 the lowerperiphery of this cover is constructed of a refractory material 25 whichcan be lowered into the molten metal bath and the airtight seal is thusaccomplished. It will be appreciated by those skilled in the art thatportions of one embodiment of the invention can be readily incorporatedinto other of the embodiments.

Although this invention has been described in its preferred form with acertain degree of particularity, it is understood that the presentdisclosure of the preferred form has been made only by way of exampleand that numerous changes in the details of construction and thecombination and arrangement of parts may be resorted to withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

lclaim:

1. An apparatus for introducing volatile or readily oxidizable additivesinto molten metal comprising a refractory-lined vessel adapted to holdmolten metal, means for introducing additives into the molten metalwithout substantially breaking the gas pressure in the vessel, conduitmeans fitted in the refractory-lined vessel below the molten metallevel, said conduit means comprising means for continuously introducinga nonreactive or inert gas into the vessel which gas bubbles up throughthe molten metal and serves to agitate the same, gas outlet means insaid vessel which is always in the open condition in operation of saidapparatus and is so adjustable as to create a small positive pressure ofthe gas above the surface of the molten metal with continuous flow ofthe gas.

2. Apparatus as claimed in claim 1 wherein said vessel is provided witha gastight cover.

3. Apparatus as claimed in claim 2 wherein said conduit means includes aporous refractory plug fitted in the vessel and through which the gasflows.

4. apparatus as claimed in claim 3, wherein inlet and outlet means areprovided for continuous introduction and removal of metal from saidvessel.

5. Apparatus as claimed in claim I, wherein the refractorylined vesselcomprises an upper part, which projects into the molten metal bath, toprovide a gastight seal.

2. Apparatus as claimed in Claim 1 wherein said vessel is provided with a gas tight cover.
 3. Apparatus as claimed in Claim 2 wherein said conduit means includes a porous refractory plug fitted in the vessel and through which the gas flows.
 4. Apparatus as claimed in Claim 3, wherein inlet and outlet means are provided for continuous introduction and removal of metal from said vessel.
 5. Apparatus as claimed in Claim 1, wherein the refractory-lined vessel comprises an upper part, which projects into the molten metal bath, to provide a gastight seal. 